1) Field of the Invention
The invention relates to a process involving heterophase reactions in a liquid or supercritical dispersion medium and also to an apparatus for carrying out the process.
2) Background Art
Heterophase reactions are chemical processes in which the reaction product is not soluble in the carrier medium but is obtained in finely divided form therein. They include not only precipitation reactions or crystallizations but, in particular, also heterophase polymerizations such as emulsion polymerizations, suspension polymerizations and dispersion polymerizations.
The starting mixture of a heterophase reaction can be either homogeneous or heterogeneous. Typical particle sizes for the reaction products of heterophase reactions are from 10 nm to a few millimeters. In most cases, unreacted starting materials have to be removed from the reaction mixture or the product after the reaction is complete. It is frequently also necessary to separate the finely divided reaction product from the reaction mixture. Heterophase reactions are usually carried out in closed stirred vessels or cascades of stirred vessels or in tube reactors. Here, both pure batch processes and semicontinuous or continuous processes are possible for the reactions.
Many of the starting materials used in heterophase reactions, in particular the monomers used in heterophase polymerizations, have a strong odor and are harmful to health. Stabilizers as well as, for example, initiator components or other polymerization auxiliaries often remain in the reaction product. This can lead to impairment of the use properties of the polymer. For this reason, these auxiliaries as well as, for example, oligomers or products of secondary reactions therefore have to be extracted from the reaction products in many cases.
Some of the substances separated from the reaction products, for example monomers or stabilizers, may, after a work-up step, be re-used in a later polymerization. This can reduce manufacturing costs and costs for disposal of waste. Many heterophase reactions including, in particular, heterophase polymerizations are carried out in an environmentally friendly dispersion medium such as water. To prevent, inter alia, the formation of waste-water, gases liquefied under high pressure or gases in the supercritical state at temperatures and pressures above the corresponding critical parameters have recently also been used as dispersion medium as alternatives to water.
A frequently employed solvent and/or dispersion medium is carbon dioxide in the liquid or supercritical state (D.A. Canelas, J. M. DeSimone: Adv. Polym. Sci. 133 (1997) 103). The supercritical state of carbon dioxide can be achieved without an excessively high engineering effort. The values for the critical point are Tcrit(CO2)=31.1xc2x0 C. and pcrit(CO2)=73.8 bar. The supercritical state of carbon dioxide is characterized by a very low viscosity and a readily variable density, by means of which the solvent properties of the carbon dioxide can also be varied. Reviews of polymerization processes in liquid and supercritical carbon dioxide may be found in A. I. Cooper, J. M. DeSimone: Curr. Opin. Solid State Mater. Sci. 1 (1996) 6. The separation of the polymer from the supercritical fluid can be carried out by means of separation processes which are known in principle (e.g. EP-B 159021).
It is an object of the present invention to provide a process and an apparatus for heterophase reactions, in particular, for dispersion polymerizations in a liquid or supercritical carbon dioxide, which offers the possibility of carrying out the reactions continuously, semi-continuously or batchwise and the possibility of concentrating the reaction product and re-using unreacted starting materials.
The invention provides a process for heterophase reactions in a liquid or supercritical dispersion medium, in which the starting materials are introduced into the reaction zone of a reactor provided with one or more enrichment zones and the reaction product or unreacted starting material or both starting material and reaction product are discharged via one enrichment zone in each case, wherein
a) the reaction product together with the dispersion medium is discharged from the reactor via an enrichment zone, the reaction product is separated off and the dispersion medium is, if desired, in countercurrent to the reaction product/dispersion medium mixture and via an enrichment zone, returned to the reaction zone, or
b) unreacted starting material together with the dispersion medium is discharged, the starting material is separated from the dispersion medium and returned directly to the reaction zone and the dispersion medium is, if desired in countercurrent to the reaction product/dispersion medium mixture and via an enrichment zone, returned to the reaction zone, or the steps a) and b) are combined with one another.